SINTERED Ag-GRAPHITE-Ni METAL CERAMIC MATERIAL FOR MAKING ELECTRIC CONTACTS AND METHOD OF PRODUCING SAME

ABSTRACT

A metal ceramic material for electrical contacts comprises 5-30 percent of silver, 0.5-50 percent of graphite, nickel - the balance. A method of producing this material consists in that a mixture of the above-mentioned components is subjected to cold pressing under a specific pressure of 2-3 t/cm2 and then to hot pressing under a specific pressure of 5-6 t/cm2 at a temperature of 400*450*C during a period of from 10 sec to 1 minute.

United States Patent [191 Filippov et al.

1 SINTERED AG-GRAPHITE-NI METAL CERAMIC MATERIAL FOR MAKING ELECTRIC CONTACTS AND METHOD OF PRODUCING SAME [22] Filed: Aug. 19, 1971 [21] Appl. No.: 173,233

[52] US. Cl 29/182.5, 75/201, 75/214,

75/226 [51] Int. Cl 1322f 3/16 [58] Field of Search 75/201, 214, 226;

29/l82.5, 630 C, 630 E, 630 R; 252/12 [451 Apr. 16, 1974 [56] References Cited UNITED STATES PATENTS 3,455,023 7/1969 Fawzy 75/201 X 3,385,677 5/1968 Schreiner et a1 29/1825 2,983,034 5/1961 Humenik et a1. 29/1825 Primary Examiner-Carl D. Quarforth Assistant Examiner-R. E. Schafer Attorney, Agent, or Firm-Eric H. Waters [5 7] ABSTRACT A metal ceramic material for electrical contacts comprises 5-30 percent of silver, 0.5-50 percent of graphite, nickel the balance.

A method of producing this material consists in that a mixture of the above-mentioned components is subjected to cold pressing under a specific pressure of 2-3 t/cm and then to hot pressing under a specific pressure of 5-6 t/cm at a temperature of 400-450C during a period of from 10 sec to 1 minute.

2 Claims, No Drawings SINTERED AG-GRAPHlTE-NI METAL CERAMIC MATERIAL FOR MAKING ELECTRIC CONTACTS AND METHOD OF PRODUCING SAME The present invention relates to the art of manufacture of electric contacts to be used for switching electric circuits in various climatic conditions, and more specifically for neutral plug micro relays of the slowaction type.

The most advantageous present invention is in various automatic and remote control arrangements, the contacts according to the invention being useful on a large scale in instruments and apparatus performing automatic control and monitoring of signalling. They may be used, for instance, in apparatus for railway traffic control, for generating diverse signals, etc., said signals being employed for automation of technological processes, to ensure safety of railway traffic, etc.

The metal ceramic material for making electric contacts is very advantageous in those relay apparatus, which are adapted to control the duration of current flow, thereby providing for decelerated opening of electric circuits, e.g., for 0.7-0.8 sec.

This metal ceramic material may be used for making electric contacts adapted to be used for closing and opening of weak-current circuits, for continuous opening and closing of AC and DC electric circuits at up to l A, for decelerated opening and closing of electric circuits and for effecting closing and opening of circuits in arrangements, wherein extinguishing of arc is provided and where there is a possibility of short-circuiting and build-up of short duration emergency current of up to 1,500 A at 30 V.

Ultimately, this metal ceramic material for making electric contacts is intended to be used in low-voltage electric equipment where very low contact resistance is required and overheating of hardware is intolerable.

Contacts to be used in electric equipment and instruments should comply with the following requirements: reliability and accuracy under conditions in which hardware is designed to operate; long service life and the absence of weldability and sintering to silver contacts with which they cooperate.

Reliability and accuracy of operation of hardware and instruments are determined by the amount of transitional resistance between said contacts, since this factor influences trouble-free and reliable operation of automatic equipment.

The amount of transitional resistance between the contacts is reduced by selecting the composition of material and using appropriate technological steps of manufacturing.

It is known that contacts having low transient resistance contribute to small heat release in equipment, thereby ensuring normal operation of hardware.

The characteristic of durability of contacts includes wear resistance determining the service life of hardware. The higher the wear resistance, the longer is the operation of hardware without additional repair involving replacement of worn contacts.

Known in the art is a metal ceramic material for making electric contacts to be used in low-voltage equipment, comprising as the main components:

field of application of the silver 50-80% graphite l4% nickel 20-45% The prior material comprises a powder which is capable of being pressed.

This prior art material exhibits the following disadvantages:

The presence of a relatively small quantity of graphite in the material results in that contacts, consisting of almost one metal silver are subject to welding and sintering at minor increase of current loads even for a short period of time.

The presence of a large quantity of silver in the composition of the material results in relatively low mechanical resistance of contacts, since silver exhibits low adhesion to graphite, whereby the service life of contact is reduced.

Furthermore, silver has low plasticity (3 units) which is also unfavorable for mechanical resistance of the contact.

A content of nickel in an amount of 20-45% does not provide for sufficient plasticity of the molding powder regardless of the high plasticity of nickel (up to 9 units).

The above disadvantages of the prior-art material for making contacts result in reduction of both reliability and durability of parts made of the prior art material.

The prior art composition of a metal ceramic contact material does not ensure the manufacturing of parts having high durability and reliability mainly due to the fact that contacts made of such material are liable to welding and sintering.

The reasons for this are the following:

With a low content of graphite, the latter does not provide for shielding metallic particles, whereby the contact is essentially metallic, that is readily weldable and liable to sintering.

Silver is, by reason of its nature, a readily weldable metal, and at high content it contributes to welding resulting in unreliance of the apparatus provided with such contacts.

Also known in the art are methods of producing a metal ceramic material, wherein a mixture of silver, graphite and nickel in powder form is successively subjected to a cold and hot pressing.

The conditions of pressing of a powder mixture are very critical in producing metal ceramic materials for making contacts, these conditions having to contribute to the attainment of high wear resistance of the material, low transitional resistance and non-weldability with other contacts. In prior art methods of producing a metal ceramic material sintering of the powder mixture of the above-said components was effected by directly heating the mixture by means of electric current during the pressing.

Hot pressing in producing material for making contacts is effected in a protective medium or atmosphere, as well as with the addition of such substances which form a reducing medium into a powder mixture of silver, graphite and nickel.

The above prior-art methods of producing a metal ceramic material do not ensure the obtainment of electric contacts having durability and resistance to welding in operation. This may be explained by the fact that it is not possible to obtain a protective atmosphere during the hot pressing and controlled supply of a protective medium into a material being pressed due to different density and plasticity of molding powders. As a result, finished contacts are very heterogeneous as to quality. In addition, it is required to use always the same quantity of metallic powders in a composition of a material for making contacts both as to plasticity and the content of carbon oxide, as well as to the presence of work hardening on the particles of metallic powders. In the absence of the above conditions, and even in case of slight deviation from specifications, it is not possible to obtain a contact material having uniform chemical and physical properties.

One of the objects of the present invention is to provide a metal ceramic material for making electric contacts having high wear resistance.

Another object of the invention is to provide a metal ceramic material for making electric contacts having low transitional resistance in pair with other electric contacts.

Still another object of the invention is to provide a metal ceramic material for making electric contacts which is non-weldable with other contacts during operation under short-duration high load (short circuit).

One of the main objects of the invention is to provide a method of producing a metal ceramic material for making electric contacts having the above-mentioned properties.

These and other objects of the invention are accomplishcd by the provision of a metal ceramic material for making electric contacts, which, according to the invention, comprises (wt.%):

silver S-30 graphite 0.5-50 nickel the balance.

This chemical composition of the material according to the invention ensures elevated wear resistance and reliability in operation as compared to prior art materials used for the same purpose.

The material according to the invention is produced by a method, wherein a mixture of said components in powder form is successively subjected to a hot and cold pressing, the cold pressing being performed, according to the invention, under a specific pressure of 2-3 t/cm whereas the hot pressing is performed under a specific pressure of 5-6 t/cm at a temperature of 400450C for a period of from sec. to 1 minute.

The invention will now be described with reference to a specific embodiment thereof.

The metal ceramic material for making electric contacts according to the invention comprises the following components: 5-30 wt. percent of silver, 0.5-50 wt. percent of graphite, nickel being the balance.

The above-listed components are used in the finely divided powder form.

In order to attain high plasticity of the material according to the invention, it is necessary to check starting powders as to moldability (plasticity), with the plasticity for graphite being not less than 5.5, silver 3.5 and nickel, 5.3.

The composition of the metal ceramic material was selected on the basis of the following considerations.

Silver has high electric and thermal conductivity and constitutes a component impeding the formation of oxide film. These positive properties of silver contributes to the obtainment of the material having very low specific resistance which remains unchanged during the entire period of operation. However, silver has a number of substantial negative properties, since it is subject to a cold welding in a large degree and is welded and relatively easy eroded under the action of an electric arc. In addition, although silver possesses sufficient strength, its wear resistance is low. In view of these properties of silver, as it has been found on the basis of research, its content in the composition of the material should not be less than 5 wt. percent and not exceed 30 wt. percent, the content of silver being variable depending on the requirements to physical properties of parts and on conditions of operation thereof.

The silver content is limited to 30 percent due to the fact that silver exhibits very low adhesion to graphites, of the order of 255 erg/cm? The adhesion or wetting power of silver with respect to graphite greatly influences mechanical strength of a contact material, and the greater the silver content in the composition of the material, the less is the mechanical strength of a contact.

While nickel possesses relatively high specific resistance, it has a low boiling point, thereby exhibiting considerable resistance to wear and formation of oxide films. In addition, nickel is not so easy to be welded as silver is, and has very important and essential advantage against silver, since it exhibits a wetting power or adhesion with respect to graphites of 2,704 erg/cm? Hence, the greater the nickel content in the composition of the material, the higher is the mechanical strength of the metal ceramic material for making contacts. Greater content of nickel results in greater wear resistance and resistance to welding and sintering.

For the material according to the invention the graphite should have an ash content of less than 5 percent, non-concentrated, crystalline, fine-grained, having at least percent and not more than percent of particles passed through a 0045 sieve. This graphite has high refractoriness and is substantially inert with respect to diverse corrosive media. Graphite has a low coefficient of expansion which fact contributes to the obtainment of parts characterized by stability of dimensions at elevated temperatures. Low modulus of elasticity, low coefficient of linear expansion and high thermal conductivity ensure thermal stability of contacts and reduce to a minimum the possibility of changes in their dimensions in operation, while cracking of contacts as a result of variation of temperature conditions during operation is also prevented due to the presence of graphite.

It is known that, when compared to pure silver, a nickel-silver composition is much more resistant to erosion and are formation at sufficiently good contact resistance. These compositions exhibit very good plasticity, are easily treated and pressed, but are liable to welding and sintering which is not favorable for reliability and durability in operation.

The present invention provides the use of the abovementioned composition nickel-graphite-silver, with the components being used in the above quantities, which certainly contributes to the obtainment of a metal ceramic material having elevated pressibility as compared to a nickel-silver composition, since the presence of graphite in a quantity of up to 50 wt. percent in the composition of the material favors an increase in its plasticity.

The composition according to the invention ensures the obtainment of such a material, which has a low specific resistance (not exceeding 1 ohm.mm/m), low transient resistance (1.10 ohm), high compressive strength (at least 450 kg/cm") and high wear resistance (9,200,000 of switching cycles under a load of 2 A at 30 V).

The metal ceramic material for making electric contacts according to the invention is produced by the following technological steps.

Starting powders are mixed in any conventional mixing means, which permits obtaining good mixing of powders and uniform distribution thereof over the entire volume.

The resulting powder mixture is subjected to pressing in a cold mold normal atmospheric conditions (room conditions) under a specific pressure of 2-3 tlcm The blanket which is thus obtained is then placed into a mold heated up to 400450C, and is pressed therein with simultaneous calibration to a required size under a specific pressure of 5-6 t/cm during a time period of from to 60 sec.

The quality of contacts made of the material according to the invention depends upon the conditions of the performance of technological steps, that is upon the temperature and specific pressure during the pressing. If the above-mentioned limits are satisfied, the contacts will have good physical and chemical, as well as operational characteristics. However, with reduction of the pressing temperature below 400C the mechanical strength of the contact material is reduced, specific resistance is increased, wear resistance is drastically reduced and transitional resistance in pair with other contacts is increased. Where the pressing temperature is increased (over 450C), overpressing takes place as a rule, which is manifested in the form of cracks, the fracture of a pressed part having a stratified nature with the feature of stratifications and other defects.

An increase or reduction of specific pressure during the pressing gives the same results. Low specific pressure of the order of less than 5 t/cm results in the obtainment of material with low mechanical strength, while a specific pressure exceeding the established limit (6 t/cm) contributes to stratification and cracking of the contact material.

Better understanding of the invention may be had from the following specific examples of embodiments thereof.

EXAMPLE 1 A powder mixture of the following composition:

wt. percent of silver,

20 wt. percent of graphite,

60 wt. percent of nickel, was pressed in a cold mold at a specific pressure of 2 t/cm to obtain a green blanket" of the material, the resulting blanket was then placed in a mold heated up to 420C and pressed therein under a specific pressure P of 5.5 t/cm for 35 sec.

The material produced on the basis of the above composition was effectively used for making contacts which were successfully employed in small-sized lowvoltage equipment with deceleratd closing and opening of electric circuit under a load of 2 A do at 30 V. The contact exhibited sufficiently low transient resistance (1.10 ohm), high wear resistance and low specific resistance (not exceeding 1 ohm.mm /m), with the wear resistance as referred to a number of switching cycles being of more than 9.2 mln.

EXAMPLE 2 A powder mixture of the following composition:

20 wt. percent of silver,

l0 wt. percent of graphite,

wt. percent of nickel, was pressed in a cold mold under a pressure of 2.5 t/cm to obtain a green blanket, the blanket was then placed in a mold heated up to 400C and pressed therein under a specific pressure P of 5 t/cm for 30 sec.

The material made by the above-said method and having the above composition was also used in lowvoltage equipment and had low transitional resistance (1.10 ohm), low specific resistance (not exceeding 0.5 ohm.mm /m) under a load of 5A do at 40 V. Under emergency load of over 200 A at 30 V the contacts were welded and fused with silver contacts. At the same time, the material according to Example 2 had elevated mechanical strength, with the compressive strength being of at least 550 kg/cm EXAMPLE 3 A powder mixture of the following composition:

25 wt. percent of silver,

30 wt. percent of graphite,

45 wt. percent of nickel, was pressed in a cold mold under a specific pressure of 3 t/cm to obtain a green blanket, the blanket was then placed in a mold heated up to 450C and pressed therein under a a specific pressure P of 6 t/cm for 60 sec.

The metal ceramic material producing by the abovementioned method and having above composition permitted to produce the contacts having elevated mechanical strength, which was, nevertheless, somewhat lower as compared to the strength of the contact according to Example 2. However, the strength of the contact according to Example 3 was of at least 350 kg/cm The material according to Example 3 had higher transitional resistance than that of Example 1. The contact material withstood emergency short-circuit of 1,500 A dc up to 200,000 switching cycles, whereas the material according to Example 1 withstood only up to 150,000 cycles.

What is claimed is:

1. A sintered metal ceramic material for making electric contacts, comprising 5-30 wt. percent of silver, 0.5-50 wt. percent of graphite, nickel the balance.

2. A method of producing a metal ceramic material for making electric contacts, comprising first subjecting a mixture of silver, graphite and nickel in powder form to a cold pressing under a specific pressure of 2-3 t/cm and then to a hot pressing under a specific pressure of 5-6 t/cm at a temperature of 400450C. during a period of from 10 seconds to 1 minute. 

2. A method of producing a metal ceramic material for making electric contacts, comprising first subjecting a mixture of silver, graphite and nickel in powder form to a cold pressing under a specific pressure of 2-3 t/cm2 and then to a hot pressing under a specific pressure of 5-6 t/cm2 at a temperature of 400*-450*C. during a period of from 10 seconds to 1 minute. 